The cellular mechanisms that regulate the production and survival of sensory receptors are not understood. In other parts of the nervous system, it is becoming increasingly evident that cells of the immune system (macrophages and other leukocytes) play a critical role in the regulation of cell production and survival, especially in response to injury. Although resident populations of leukocytes have been identified in the sensory organs that mediate hearing, balance1 taste, and smell, little is known about how the immune system influences the development and maintenance of any of these organs. The proposed experiments are aimed at enhancing our present understanding of the interaction between cells of the immune system and the sensory receptors of the nervous system. Initial experiments will examine the influence of resident leukocytes on cell addition and cell death in the vestibular organs of birds. Additional experiments will determine the influence of the immune system and cytokines on hair cell regeneration in the avian cochlea in vivo and in isolated sheets of supporting cells Finally, a series of studies will examine the regeneration of inner ear sensory cells and chemoreceptive cells in mice that possess mutations that result in rnacrophage deficits.